Pierce the chicken breast with a fork about 10 times. Place it in a heatproof container, add sake, and rub it in. Cover loosely with plastic wrap and microwave at 600W for 5 minutes. Let it sit covered for 10 minutes to finish cooking with residual heat.

Smash the cucumbers with a rolling pin, trim the ends, and cut them into bite-sized pieces. Grate the onion and place it in a separate heatproof container. Microwave at 600W for 1 minute without covering, then let it cool.

Mix the grated onion with sugar, soy sauce, sesame oil, lemon juice, dashi powder, and salt and pepper to create the onion soy dressing.